This invention relates to a power steering apparatus for an automobile or other vehicle. More particularly, it relates to a power steering apparatus having an improved mechanism for detecting when the driver of the vehicle is imparting torque to the steering wheel of the vehicle.
In a power steering apparatus for an automobile, when the driver applies torque to the steering wheel of the automobile, the steering shaft is caused to undergo an initial slight rotation. The rotation is detected by a rotation detecting mechanism, and a motor is energized in response to this initial rotation to rotate the steering shaft and decrease the force which must be exerted by the driver. Various mechanisms have been proposed for detecting the rotation of steering shafts, including those using strain gauges and induction potentiometers.
For example, Japanese Laid Open Patent Application No. 46-35779 discloses a power steering apparatus in which the torque applied to a steering shaft is detected by strain gauges disposed on a rotating shaft which is disposed in parallel to the steering shaft and which is driven by the steering shaft through gears. However, strain gauges are difficult to install and calibrate, and have the further disadvantage of being sensitive to changes in temperature.
Japanese Patent No. 1038166 discloses a power steering apparatus in which the rotation of a steering shaft produces a change in the magnetic resistance of a magnetic circuit comprising a metallic bracket mounted on the steering shaft and a stationary pole which surrounds the steering shaft. The change in magnetic resistance produces a change in the inductance between a primary coil and a secondary coil wrapped around the bracket. An alterhating current is applied to the primary coil, and the change in the induced voltage in the secondary coil as a result of the change in inductance is measured to detect the rotation of the steering shaft. However, that invention has the disadvantage that the output of the secondary coil is dependent on the speed of rotation of the steering shaft and is not stable. Furthermore, the output of the secondary coil, which rotates with the steering shaft, is input to a stationary member via an output lead which is wrapped around the steering shaft. There is the thus the possibility of the output lead becoming entangled with the steering shaft or other member and being severed.